High-frequency electric field coupler, communication system, and communication apparatus

ABSTRACT

A high-frequency electric field coupler is provided for use in a contactless communication system propagating high frequency signals in a wide band utilizing electric field coupling. The coupler includes a coupling electrode and a resonance unit operating to increase the amplitude of an electrical charge accumulated at the coupling electrode at a predetermined resonance frequency. The coupling electrode is a linear conductor in the form of a coil.

CROSS REFERENCES TO RELATED APPLICATIONS

The present invention contains subject matter related to Japanese PatentApplication JP 2007-268018 filed in Japanese Patent Office on Oct. 15,2007, the entire contents of which being incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high-frequency electric field couplerused in a contactless communication system propagating high-frequencysignals in a wide band utilizing electric field coupling and relates toa communication system and communication apparatus utilizing thecoupler. More particularly, the invention relates to a high-frequencyelectric field coupler with excellent characteristics having a couplingelectrode and a resonance unit operating to increase the amplitude ofelectrical charge accumulated at the coupling electrode at apredetermined resonance frequency, and the invention relates to acommunication system and a communication apparatus utilizing thecoupler.

2. Description of the Related Art

Most radio communication systems according to the related art such asradio LANs (local area networks) which are typically in compliance withIEEE802.11 and Bluetooth (a registered trademark) communication systemsare electrical wave communication systems which propagate a signalutilizing a radiation electric field generated when a current is passedthrough an aerial (antenna). A communication system of this type has aproblem in that a transmitting end of the system can be a source of anelectric wave disturbing an adjacent communication system because anelectric wave may be radiated regardless of whether there is a party tocommunicate.

Methods of radio communication other than electric wave communication asdescribed above include contactless communication methods which utilizean electrostatic field or induction field. For example, existingcontactless communication systems primarily used for RFID (radiofrequency identification) utilize the electric field coupling method ormagnetic field coupling method. An electrostatic field undergoes anabrupt attenuation inversely proportionate to the third power of thedistance of the same from the source, and an induction field undergoesan abrupt attenuation inversely proportionate to the second power of thedistance of the same from the source. Therefore, such a field allowsradio communication to be performed at such an extremely low power thatthe intensity of an electrical field (the intensity of an electric wave)at a distance of 3 meters from a radio facility equals or falls below apredetermined level. No license is required to run a radio station insuch a case. In the case of a contactless communication system of thistype, no coupling relationship occurs and no electric field is radiatedwhen the system has no party to communicate in the neighborhood of thesame. Therefore, the system will not disturb any other communicationsystem. Even when there is an incoming electric wave from a remotesource, an electric field coupler of the system will not receive theelectric wave. Thus, the system will not be interfered by any othercommunication system.

There are proposals on RFID tag systems including a set of communicationaiding bodies which are disposed such that an RFID tag is locatedbetween a plurality of communication aiding bodies. RFID tags attachedto a plurality of commodities are disposed such that they are sandwichedby communication aiding bodies. Thus, information can be reliably readfrom the RFID tags or written in the tags even when the tags overlapeach other (See JP-A-2006-60283 (Patent Document 1 ) for example).

SUMMARY OF THE INVENTION

An RFID system utilizing an electrostatic field or induction field isnot suitable for transmission of a large volume of data because it useslow frequency signals which result in a low communication speed. In thecase of a communication system utilizing an induction field generated byan antenna coil, communication is disabled when there is a metal platebehind the coil. Problems therefore arise in packaging the system,including a need for disposing the coil on a plane having a largesurface area. The system also suffers from significant losses in thetransmission path and consequently has low efficiency, of signaltransmission.

Under the circumstance, the inventors thought that a contactlesscommunication system capable of high speed data transmission could beprovided by allowing high frequency signals in a wide band to bepropagated using electric field coupling. Obviously, such a contactlesscommunication system having a higher speed requires no license when itis used as a radio station because the system utilizes an extremely weakelectric field, and secrecy can be also sufficiently guaranteed.

In a contactless communication system utilizing electric field coupling,one capacitor is formed by a coupling electrode at a transmitting endand a coupling electrode at a receiving end, the electrodes beingprovided in the form of sheets facing each other. Since the system as awhile operates like a band-pass filter, a high-frequency signal can beefficiently transmitted between the two high-frequency electric fieldcouplers.

It is known that the capacity of a capacitor is proportionate to thesurface area of electrodes provided to face each other. A problemtherefore arises in that it is difficult to make coupling electrodescompact when consideration is paid to a need for generating an effectiveelectric field which allows communication to be established betweencouplers of a transmitter and a receiver. It is assumed here thatprimary applications of a contactless communication system are smallinformation terminals such as cellular phones and digital still cameras,and the size of coupling electrodes becomes a bottleneck in designingsuch apparatus.

It is desirable to provide a high-frequency electric field coupler to beused in a contactless communication system propagating high-frequencysignals in a wide band utilizing electric field coupling and to providea communication system and a communication apparatus utilizing such acoupler.

It is also desirable to provide an excellent high-frequency electricfield coupler with high characteristics having a coupling electrode anda resonance unit operating to increase the amplitude of electricalcharge accumulated at the coupling electrode at a predeterminedresonance frequency and to provide a communication system and acommunication apparatus utilizing such a coupler.

It is also desirable to provide an excellent high-frequency electricfield coupler which can be made compact while maintaining sufficientcharacteristics of coupling with another coupler of a party tocommunicate at a predetermined resonance frequency. It is also desirableto provide a communication system and a communication apparatusutilizing such a coupler.

The invention was made taking the above-described problem intoconsideration. According to an embodiment of the invention, there isprovided a high-frequency electric field coupler used in a contactlesscommunication system propagating high-frequency signals in a wide bandutilizing electric field coupling, characterized in that it includes acoupling electrode and a resonance unit operating to increase theamplitude of electrical charge accumulated at the coupling electrode ata predetermined resonance frequency and in that the coupling electrodeis a coiled linear conductor.

The high-frequency electric field coupler according the embodiment ofthe invention can be made very small because the coupling electrode is alinear conductor which is bent in the form of a coil. Just as done inhigh-frequency electric field couplers according to the related art, thecoupling electrode is connected to the resonance unit which operates toincrease the amplitude of electrical charge accumulated at the couplingelectrode at a predetermined resonance frequency. Thus, thehigh-frequency electric field coupler can be provided with highcharacteristics.

The surface of the linear conductor may be coated with an insulationlayer to prevent shorting from occurring in the middle of the coil.Since the coil has low mechanical strength without a core, the linearconductor may be wound around a core bar. Alternatively, after theconductor is wound in the form of a coil, it may be fixed using a resinto improve the strength of the same.

The resonance unit may be provided in the form of a lumped constantcircuit or a distributed constant circuit, i.e., a stub. When a stub isused, the stub may be an open stub whose terminating end is open or ashort stub whose terminating end is shorted to ground. In either of theconfigurations, circuit constants or stub length is set such that theresonance unit will have the function of operating to increase theamplitude of electrical charge accumulated at the coupling electrode ata predetermined resonance frequency.

For example, the high-frequency electric field coupler is formed byshorting an end of the coupling electrode, which is a coiled conductor,to ground and connecting the resonance unit, which is a lumped constantcircuit such as a capacitor element, in parallel with the couplingelectrode.

Alternatively, the resonance unit may be a short stub having a lengththat is one-half of a used wavelength λ. The high-frequency electricfiled coupler may be formed by connecting the coupling electrode, whichis a coiled conductor, to a position of the short stub substantially atthe center thereof and shorting an end of the coupling electrode to theground.

Alternatively, the resonance unit may include a first resonance stub anda second resonance section which are substantially equal to each otherin length. The resonant unit is an open stub having a terminating end inan open state, and both ends of the coupling electrode in the form of acoiled conductor are connected to the first and second resonance stubs,respectively to form a high-frequency electric field coupler. In thiscase, the combined length of the coiled coupling electrode and the firstand second resonance stubs is preferably substantially equivalent to afull wavelength, i.e. the entirely of the used wavelength λ.

The coupling electrode in the form of a coiled linear conductor may bedisposed to extend straightly and may be connected to the resonance unitin such a state. As a modification to this, the electrode may be bent inthe form of a ring. In the latter case, since the coupling electrode canbe accommodated in a smaller space because of the ring-like shape, afurther reduction can be achieved in the size of the high-frequencyelectric field coupler. While the straight coil generates an electricfield uniformly oriented in the radial direction of the coil, thering-shaped coil provides stable coupling strength because it is lessorientation-dependent. It is considered that a size reduction and animprovement in coupling strength can be more significantly achieved bybending the coil serving as a coupling electrode in a helical shapeinstead of a ring-like shape.

When a coupling electrode in the form of a coiled conductor bent in aring-like shape is used, the winding direction of the coil may bereversed in the middle of the ring. The result is equivalent to forminga coupling electrode in the form of a coil by combining a plurality ofcoils wound in opposite directions. A magnetic current generated in thecoupling electrode can be cancelled to reduce radiation of unnecessaryelectric waves by reversing the winding direction of the coil.

According to the embodiment of the invention, an excellenthigh-frequency electric field coupler to be used in a contactlesscommunication system for propagating high-frequency signals in a wideband utilizing electric field coupling can be provided along with acommunication system and a communication apparatus utilizing the same.

According to the embodiment of the invention, it is possible to providean excellent high-frequency electric field coupler with highcharacteristics having a coupling electrode and a resonance unitoperating to increase the amplitude of electrical charge accumulated atthe coupling electrode at a predetermined resonance frequency along witha communication system and a communication apparatus utilizing the same.

According to the embodiment of the invention, an excellenthigh-frequency electric field coupler of small size can be providedwhile maintaining sufficient characteristics of coupling of the samewith a coupler of a party to communicate at a predetermined resonancefrequency. A communication system and a communication apparatusutilizing such a coupler can be also provided. According to theembodiment of the invention, a coupling electrode may be formed using alinear conductor and may be bent in the form of a coil, which allows asignificant size reduction. A resonance unit may be connected to thecoupling electrode to increase the amplitude of electrical chargeaccumulated at the coupling electrode at a predetermined resonancefrequency just as done in high-frequency electric field couplersaccording to the related art. Thus, a high-frequency electric filedcoupler having high characteristics can be provided.

Other objects, features and advantages of the invention will becomeapparent from the following detailed description of the invention basedon the embodiment and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a configuration of a contactlesscommunication system based on electric field coupling using anelectrostatic field or induction field;

FIG. 2 shows an equivalent circuit of a high-frequency electric fieldcoupler;

FIG. 3 shows an equivalent circuit of the contactless communicationsystem shown in FIG. 1;

FIG. 4 shows an example of a configuration of a high-frequency electricfield coupler mounted on a printed board;

FIG. 5 shows a configuration of a high-frequency electric field couplerhaving a resonance unit that is a lumped constant circuit;

FIG. 6 shows an example of a configuration of a high-frequency electricfield coupler having a resonance unit that is a half-wavelength stub;

FIG. 7 shows an example of a configuration of a high-frequency electricfield coupler having a resonance unit that is an open stub;

FIG. 8 shows another example of a configuration of a high-frequencyelectric field coupler having a resonance unit that is an open stub;

FIG. 9 shows another example of a configuration of a high-frequencyelectric field coupler having a resonance unit that is a half-wavelengthshort stub; and

FIG. 10 is a graph showing results of a comparison between propagationlosses of a high-frequency electric field coupler having a couplingelectrode that is a coiled linear conductor and propagation losses of ahigh-frequency electric field coupler having a sheet-like couplingelectrode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the invention will now be described in detail withreference to the drawings.

The invention relates to a communication system for transmitting databetween information apparatus utilizing an electrostatic field orinduction field. When a communication method based on an electrostaticfield or induction field is used, no coupling relationship exists and noelectric wave is radiated when a communication system of interest has noparty to communicate in the vicinity of the same. Therefore, thecommunication method will not result in any disturbance to othercommunication systems unlike an electrical wave communication methodutilizing a radiation electrical field. Even when there is an incomingelectric wave from a remote source, the electric wave is not received bya coupler of the communication system. Thus, the system is notinterfered by other communication systems.

In the case of electrical wave communication using an antenna accordingto the related art, the field intensity of a radiation electric field isinversely proportionate to the distance of the field from the source.The field intensity of an induction field attenuatesinverse-proportionately to the second power of the distance of the fieldfrom the source, and an electrostatic field attenuatesinverse-proportionately to the third power of the distance of the fieldfrom the source. According to a communication method based on electricfield coupling, it is possible to form a radio system having extremelylow power which constitutes merely a noise level for other radio systemsexisting in the neighborhood. There is no need for obtaining a licensefor such a radio station.

An electrostatic field fluctuating with time is sometimes called aquasi-electrostatic field, and such an electric field will be alsoreferred to as “an electrostatic field” in this specification.

Communication utilizing an electrostatic field or induction fieldaccording to the related art is not suitable for transmission of a greatamount of data because low-frequency signals are used in suchcommunication. On the contrary, a communication system according to anembodiment of the invention can transmit a great amount of data becausethe system transmits high-frequency signals utilizing electric fieldcoupling. Specifically, high volume data communication can be performedthrough extremely weak radio transmission by applying a communicationmethod using high frequencies and a wide band such as UWB (ultra wideband) communication to electric field coupling.

The inventors consider a data transmission system utilizing low-band UWBto be one of radio communication techniques which are advantageous whenimplemented in mobile apparatus. For example, such a technique allowshigh speed data transmission in a short range such as that achieved withan ultra high speed short range DAN (device area network) including astorage device. A UWB communication system utilizing an electrostaticfield or induction field is capable of data communication using anextremely weak electric field. In addition, such a system can transfer agreat amount of data such as dynamic images or music data of one CD at ahigh speed in a short time (for example, see the specifications ofJapanese Patent Application Nos. 2006-245614, 2006-314458, and2007-159629 which have already been assigned to the applicant).

FIG. 1 schematically shows a configuration of a contactlesscommunication system utilizing an electrostatic field or inductionfield. The illustrated communication system includes a transmitter 10which transmits data and a receiver 20 which receives data. Whenhigh-frequency electric field couplers of the transmitter and thereceiver are disposed so as to face each other as shown in the figure,two electrodes operate as one capacitor, and they operate like aband-pass filer as a whole. Therefore, a high-frequency signal can beefficiently transmitted between the two high-frequency electric fieldcouplers.

Electrodes 14 and 24 for transmission and reception provided at thetransmitter 10 and receiver 20, respectively, are disposed opposite toeach other at an interval of, for example, about 3 cm to allow electricfield coupling between them. When there is a request for transmissionfrom a host application, a transmission circuit unit 11 of thetransmitter generates a high-frequency transmission signal such as a UWBsignal based on transmission data, and the signal is propagated from thetransmitting electrode 14 to the receiving electrode 24. The receptioncircuit unit 21 of the receiver 20 demodulates and decodes the receivedhigh-frequency signal and passes the data thus reproduced to the hostapplication.

In the UWB communication system utilizing an electrostatic field, whenthere is impedance mismatch at the coupling section between thetransmitting electrode 14 and the receiving electrode 24, a signal willundergo reflection and suffer from a propagation loss. In order to copewith this problem, as shown in the equivalent circuit in FIG. 2, thetransmitter 10 and the receiver 20 are provided with respectivehigh-frequency electric field couplers which are formed by connectingrespective elements, i.e., the sheet-like electrodes 14 and 24, seriesinductors 12 and 22, and parallel inductors 13 and 23 to ahigh-frequency signal transmission path. When such high-frequencyelectric field couplers are disposed to face each other as shown in theequivalent circuit in FIG. 3, the two electrodes operates as onecapacitor, and they operate like a band-pass filter as a whole.Therefore, a high-frequency signal can be efficiently transmittedbetween the two high-frequency electric field couplers. Thehigh-frequency signal transmission path may be a coaxial cable, a microstrip line, or a coplanar line.

FIG. 4 shows an example of a configuration of the high-frequencyelectric filed coupler mounted on a printed circuit board. In theillustrated example, a high-frequency electric field coupler is disposedon a printed circuit board 101 having a ground conductor 102 formed on abottom surface thereof and a printed pattern on a top surface thereof. Aconductor pattern or stub 103, which is a distributed constant circuit,is formed instead of parallel and series inductors, the stub serving asan impedance matching unit and a resonance unit of the high-frequencyelectric field coupler. The stub is connected to atransmission/reception circuit module 105 through a signal line pattern104. An end of the stub 103 is connected or shorted to the ground 102 onthe bottom surface via a through hole 106 extending through the printedcircuit board 101, and the stub 103 is connected to a coupling electrode108 through a metal line 107 substantially in the middle thereof.

The length of the stub 103 is approximately equivalent to one-half ofthe wavelength of a high frequency signal, and the signal line 104 andthe stub 103 are formed as micro strip lines or coplanar lines on theprinted circuit board 101. When the length of the stub 103 is equivalentto a half-wavelength and the stub is shorted at one end thereof, thevoltage amplitude of a standing wave generated in the stub 103 is 0 atthe end of the stub, and there is a maximum amplitude at the center ofthe stub or the position at a distance equivalent to aquarter-wavelength from the end of the stub 103. The coupling electrode108 is connected through the metal line 107 to the center of the stub103 where the voltage amplitude is at the maximum. Thus, ahigh-frequency electric field coupler having high propagation efficiencycan be fabricated. Since an impedance matching unit is constituted bythe stub 103 which is a distributed constant circuit formed by aconductor pattern on the printed circuit board 101, uniformcharacteristics can be achieved throughout a wide band.

In the contactless communication system utilizing an electrostatic fieldor induction field, the high-frequency electric field coupler achievescoupling using an electrostatic field or induction field which undergoesdistance-dependent attenuation greater than that of a radiation electricfield radiated from an antenna. Therefore, the system is preferably usedas a coupling unit for short-range communication.

From the viewpoint of compactness, it is not preferable to form thecoupling electrode of the high-frequency electric field coupler using aplanar conductor as shown in FIG. 4.

A significant size reduction can be achieved by forming the couplingelectrode using a linear conductor and by bending the linear conductorin the form of a coil. Just as done in the high-frequency electric fieldcoupler shown in FIG. 4, a resonance unit is connected to the couplingelectrode to increase the amplitude of electrical charge accumulated atthe coupling electrode at a predetermined resonance frequency. Thus, ahigh-frequency electric field coupler having high characteristics can beprovided.

When the coupling electrode is formed using a linear conductor, thecoupling electrode and the resonance unit are provided on a printedcircuit board, and the opposite surface of the board is covered withground, just as done in the high-frequency electric field coupler shownin FIG. 4.

The coupling electrode formed using a linear conductor is a coiledconductor having an effective length which is about one-half of a usedwavelength λ. Specifically, the length is determined such that there isa phase difference of about 180 degrees between input and output signalswhich are a high-frequency signal input from one end of the coiledconductor serving as the coupling electrode and a resultant signaloutput from another end of the conductor, respectively. When ahigh-frequency signal is input, a standing wave is generated in thecoil.

The coil is prevented from shorting in the middle of the same by coatingthe surface of the linear conductor with an insulation layer. Since thecoil has low mechanical strength without a core, the mechanical strengthof the linear conductor may be improved by winding the conductor arounda core bar or fixing the conductor with a resin after winding it in theform of a coil (neither approach is illustrated).

A resonance unit is connected to the coupling electrode in the form of acoil. The resonance unit may be provided in the form of a lumpedconstant circuit or a distributed constant circuit, i.e., a stub. When astub is used, the stub may be an open stub whose terminating end is inan open state or a short stub whose terminating end is shorted toground. In either of the configurations, circuit constants or stublength is set such that the resonance unit will have the function ofoperating to increase the amplitude of electrical charge accumulated atthe coupling electrode at a predetermined resonance frequency.

FIG. 5 shows a configuration of a high-frequency electric field couplerhaving a resonance unit which is a lumped constant circuit. An end of acoupling electrode in the form of a coil is shorted to ground. Theresonance unit is a capacitor element serving as a lumped constantcircuit, and the unit is connected in parallel with the couplingelectrode.

FIG. 6 shows an example of a configuration of a high-frequency electricfield coupler having a resonance unit which is a half-wavelength shortstub. A coupling electrode in the form of a coil is connected to aposition of the short stub substantially at the center thereof, and anend of the electrode is shorted to ground. The reason for connecting thecoupling electrode to the substantially central position of the shortstub is that a standing wave in the short stub with a length equivalentto a half-wavelength has a maximum amplitude in the vicinity of thecentral position.

FIG. 7 shows an example of a configuration of a high-frequency electricfield coupler having a resonance unit which is an open stub. In theillustrated example, the resonance unit is divided into first and secondresonance stubs having substantially equal lengths, and both ends of acoupling electrode, which is a coiled conductor, are connected to thefirst and second stubs, respectively. In this case, the combined lengthof the coiled coupling electrode and the first and second resonancestubs is preferably substantially equivalent to a full wavelength or theentirety of a used wavelength λ.

FIG. 8 shows another example of a configuration of a high-frequencyelectric field coupler having a resonance unit which is an open stub. Inthe illustrated example, the resonance unit is also divided into firstand second resonance stubs substantially equal to each other in. In theexample shown in FIG. 7, the coupling electrode in the form of a coiledlinear conductor extends straightly, and both ends of the electrode areconnected to the first and second resonance stubs, respectively. On thecontrary, in the example shown in FIG. 8, the coupling electrode in theform of a coiled linear conductor is bent in the form of a ring, andboth ends of the electrode are connected to the first and secondresonance stubs, respectively. The combined length of the coiledcoupling electrode and the first and second resonance stubs ispreferably substantially equivalent to a full wavelength. In the lattercase, the coupling electrode can be accommodated in a smaller spacebecause it is in the form of a ring, and the high-frequency electricfield coupler can therefore be provided in a smaller size. While thestraight coil generates an electric field which is uniformly oriented inthe radial direction of the coil, the ring-shaped coil provides stablecoupling strength which is less orientation-dependent.

It is considered that such an effect is further improved by bending thecoil serving as the coupling electrode in a helical shape (not shown)instead of a ring-like shape.

FIG. 9 shows another example of a configuration of a high-frequencyelectric field coupler having a resonance unit which is ahalf-wavelength short stub. A coupling electrode in the form of a coilis connected to a position of the short stub substantially at the centerof the same, and an end of the electrode is shorted to ground. Thereason for connecting the coupling electrode to the substantiallycentral position of the short stub is that a standing wave in the shortstub with a length equivalent to a half-wavelength has a maximumamplitude in the vicinity of the center. The example is different fromthe example shown in FIG. 6 primarily in that the coupling electrode,which is a coiled linear conductor, is bent in the form of a ring. Thecoupling electrode can be accommodated in a smaller space because it isring-shaped, and a further reduction in the size of the high-frequencyelectric field coupler can be achieved. While a straight coil generatesan electric field which is uniformly oriented in the radial direction ofthe coil, the ring-shaped coil provides stable coupling strength whichis less orientation-dependent. It is considered that such an effect isfurther improved by bending the coil serving as the coupling electrodein a helical shape (not shown) instead of a ring-like shape.

In the example shown in FIG. 9, in the coupling electrode that is acoiled conductor bent in the form of a ring, the winding direction ofthe coil is reversed in the middle of the ring. The result is equivalentto forming a coupling electrode in the form of a coil by combining aplurality of coils wound in opposite directions. By reversing the coilwinding direction as thus described, a magnetic current generated in thecoupling electrode can be cancelled to reduce radiation of unnecessaryelectric waves.

Obviously, the reversal of a coil winding direction in the middle of aring may be carried out also in the high-frequency electric fieldcoupler having a resonance unit that is an open stub, shown in FIG. 8.

FIG. 10 shows results of a comparison between propagation losses of ahigh-frequency electric field coupler having a coupling electrodeconstituted by a coiled linear conductor and propagation losses of ahigh-frequency electric field coupler having a sheet-like couplingelectrode. The high-frequency electric field coupler having a couplingelectrode in the form of a metal sheet suffers from greater reductionsin efficiency and greater losses when the surface area and height of theelectrode are small. On the contrary, the high-frequency electric fieldcoupler having a coupling electrode in the form of a coil has smallerlosses compared to the sheet-like high-frequency electric field couplereven though it has a smaller electrode. This is considered attributableto the fact that an electric field is more easily formed outside thelinear electrode which is more weakly coupled to ground than thesheet-like electrode.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

In present specification, the description has focused on an embodimentof a coupler applied to a communication system for cableless datatransmission utilizing electric field coupling of UWB signals. However,the gist of the invention is not limited to such an embodiment. Forexample, the invention may be similarly applied to communication systemsutilizing high-frequency signals other than UWB communication systemsand communication systems transmitting data using signals havingrelatively low frequencies on an electric field coupling basis.

The invention has been disclosed in the form of presentation ofexamples, and the contents of this specification should not be taken ina limiting sense. The gist of the invention should be understood withreference to the appended claims.

1. A high-frequency electric field coupler used in a contactlesscommunication system propagating high frequency signals in a wide bandutilizing electric field coupling, comprising: a coupling electrode; anda resonance unit operating to increase the amplitude of electricalcharge accumulated at the coupling electrode at a predeterminedresonance frequency, wherein the coupling electrode is formed by acoiled linear conductor.
 2. A high-frequency electric field coupleraccording to claim 1, wherein the coupling electrode formed by a coiledlinear conductor has an effective length substantially equivalent to onehalf of a used wavelength.
 3. A high-frequency electric field coupleraccording to claim 1, wherein the length of the coupling electrodeformed by a coiled linear conductor is determined such that there is aphase difference of about 180 degrees between input and output signalswhich are a high-frequency signal input from one end of the linearconductor and a resultant signal output from another end of the same,respectively.
 4. A high-frequency electric field coupler according toclaim 1, wherein the surface of the linear conductor is coated with aninsulation layer.
 5. A high-frequency electric field coupler accordingto claim 1, wherein the linear conductor is wound around a core bar. 6.A high-frequency electric field coupler according to claim 1, whereinthe linear conductor is fixed with a resin after being wound in the formof a coil.
 7. A high-frequency electric field coupler according to claim1, wherein the resonance unit is formed by a lumped constant circuit. 8.A high-frequency electric field coupler according to claim 1, whereinthe resonance unit is formed by an open stub having a terminating end inan open state.
 9. A high-frequency electric field coupler according toclaim 1, wherein the resonance unit is formed by a short stub having aterminating end which is grounded.
 10. A high-frequency electric fieldcoupler according to claim 1, wherein: an end of the coupling electrodeformed by a coiled linear conductor is shorted to ground; and theresonance unit formed by a lumped constant circuit is connected inparallel with the coupling electrode.
 11. A high-frequency electricfield coupler according to claim 1, wherein: the resonance unit is ashort stub having a length equivalent to one half of the used wavelengthand having a terminating end shorted to ground; and the couplingelectrode formed by a coiled linear conductor is connected to a positionof the short stub substantially at the center thereof and shorted toground at an end thereof.
 12. A high-frequency electric field coupleraccording to claim 1, wherein: the resonance unit is an open stub formedby a first resonance stub and a second resonance section havingsubstantially equal lengths and having a terminating end in an openstate; both ends of the coupling electrode formed by a coiled linearconductor are connected to the first and second resonance sections,respectively; and the combined length of the coupling electrode in theform of a coil and the first and second resonance stubs is substantiallyequivalent to a full wavelength or the entirety of a used wavelength.13. A high-frequency electric field coupler according to claim 12,wherein the coupling electrode formed by a coiled linear conductor isbent in a ring-like shape or helical shape and is connected to the firstand second resonance sections at both ends thereof.
 14. A high-frequencyelectric field coupler according to claim 11, wherein the couplingelectrode formed by a coiled linear conductor is bent in a ring-likeshape or helical shape, the electrode being connected to a substantiallycentral position of the short stub and shorted to ground at an endthereof.
 15. A high-frequency electric field coupler according to claim13 or 14, wherein the winding direction of the ring-shaped coiled linearconductor forming the coupling electrode is reversed in the middle ofthe ring.
 16. A communication system propagating high-frequency signalsin a wide band between a transmitter and a receiver utilizing electricfield coupling, wherein at least either of the transmitter and thereceiver comprises a high-frequency electric field coupler including acoupling electrode formed by a coiled linear conductor and a resonanceunit operating to increase the amplitude of electrical chargeaccumulated at the coupling electrode at a predetermined resonancefrequency.
 17. A communication apparatus comprising: a communicationcircuit unit processing a high-frequency signal for data transmission;and a high-frequency electric field coupler performing electric fieldcoupling with a party to communicate provided opposite to the same at anultra short distance, wherein the high-frequency electric field couplerincludes a coupling electrode formed by a coiled linear conductor and aresonance unit operating to increase the amplitude of electrical chargeaccumulated at the coupling electrode at a predetermined resonancefrequency.